CT systems and methods are widely used, particularly for medical imaging and diagnosis. CT systems generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side thereof. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
Photon-counting detectors (PCDs) are configured to acquire the spectral nature of the X-ray source, rather than the energy integration nature acquired by conventional CT detectors. To obtain the spectral nature acquired by the conventional CT detector, the photon-counting detectors split the X-ray beam into its component energies or spectrum bins, and count a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as “spectral CT.” Since spectral CT involves the detection of transmitted X-rays at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Semiconductor-based photon-counting detectors used in spectral CT can detect incident photons and measure photon energy for each event. However, due to the interaction depth and the ballistic deficit, the measured photon energy cannot be related to incident photon energy uniquely. Moreover, a high flux can cause pileup and polarization, which ultimately causes the photon-counting detector to saturate. When charges are trapping inside the material in polarization, the internal electronic field of the material is changed and the recovery time of the PCD is extended. Therefore, currently available photon-counting detectors still require solutions to resolve issues related to polarization, in order to increase the accuracy of measuring the photon energy.